Covering cold frames


I seeded lettuces and spinach in a cold frame. It is protected all around with straw, Should I put a blanket on top when temperature dips below 0*C , or, at what lower temperature? We live near Markham ON


How nice that you have a cold frame, which creates a little microclimate in your garden.  Cold frames trap the sun’s heat, heating the soil so that plants can grow beyond the growing season, and you can also get a few weeks’ head start on the gardening season next spring.

It’s best to insulate or draft-proof the cold frame.  You are already using straw, which has great insulating properties.  Make sure the layer of straw is quite thick, around 60 cm (2 ft) and don’t pack it down.  Snow that falls on or around the frame will also act to insulate it.  However, don’t allow too much snow to remain on the frame lid as it would prevent sunlight from getting, not to mention the risk that lots of heavy snow could break the glass/plastic lid.

There is no specific temperature at which it’s recommended that extra insulation is needed.  This will depend on the type of cold frame you have, where it is situated and other factors.  However, it makes sense, when very cold nights are anticipated –  e.g., when temperatures drop below –7 degrees C (20 degrees F)  that extra insulation would be needed:

  • Inside the cold frame, additional straw, shredded leaf mulch or floating row covers can be used.
  • Cover the lid of the cold frame in order to keep the heat from escaping –use layers of blankets, towels, bubble wrap, burlap, carpet or even newspaper. These can be held down by heavy pieces of wood.    In northerly climates like ours, cold frames are often partially sunk into the soil, a strategy that improves insulation even more.
  • Be sure to remove these extra layers of insulation during the day to permit sun to enter the structure and heat the ground/air inside the frame.

The cold frame also needs good ventilation – so keep an eye on the weather each day and adjust the lid based on the weather that is anticipated.  Prop open the glass/plastic lid 5-10 cm (2-4 inches) to permit fresh air to enter the interior, if the temperature outdoors is 2-7 degrees C (35 – 45 degrees F) and sunny. If the temperature is higher, e.g.,  7-10 degrees C (45-50 degrees F) you can even remove the lid for a few hours – replace it by late afternoon in order to trap the heat inside the frame.  The temperature inside the frame should not get above 16 degrees C (60 degrees F).

All the best with your winter garden!

November 23 2020